Charging concerns/questions with NMP Lithium Ion batteries

[Lees,John W]

Troy:

I measured the output voltage on a NMP charger and got 8.89 Volts (no load
other than the impedance of the meter)--john lees 

-----Original Message-----
From: Troy Newman [mailto:troy_newman at msn.com]
Sent: Friday, May 07, 2004 8:03 AM
To: discussion at nsrca.org
Subject: RE: Charging concerns/questions with NMP Lithium Ion batteries



Keith,

I can talk about the NMP stuff. I have been using it for a long time. It 
works very well and was designed to not have the problems with other 
systems.

As for over charging using the NMP packs and charger. It is not possible. 
The charger is basically a voltage regulator that is set to 8.4V (2 cells 
means 4.2V per cell)

The way the charger works is the pack is say 7.4V and the charger is at 8.4V

this means current will flow from the charger to the pack. When the pack 
reaches its goal voltage...(the setting of the charger) no current will 
flow. Therefore it will not overcharge. The charger's electronics set the 
voltage it is not user adjustable.

Now if you will look closely at the AMA warning it says they recommend a 
dedicated charger to the specific type and brand of pack. This means no 
variables, no charger picking cell counts and so on. No picking charge 
rates.... The NMP charger is exactly this technology. It will only charge 
the 2 cell packs. There is no charge rates to change as you can't change 
them.....The pack can only recieve so much energy at once and the design of 
the charger is limited by the battery capacity to accept current. If the 
battery is at 8.1V it can't take as much current as when it was at 7.1V...so

the charger only delivers what the battery asks for.

Another analogy is a pair of balloons....ballon #1 is at 8.4psi and is a 
certain volume....Balloon #2 is 7psi.

The two ballons are attached by a hose and a valve....When the air valve is 
opened...Balloon #1 will try to inflate #2 right.....If both ballons are 
equal no pressure will flow between the 2 right?

Same thing here....as the voltage comes up on the battery pack the charger 
will not deliver the current.

This NMP battery system is exactly what the AMA is recommending for using 
Lithium batts.

Troy Newman

>From: "Keith Black" <tkeithb at comcast.net>
>Reply-To: discussion at nsrca.org
>To: <discussion at nsrca.org>
>Subject: Charging concerns/questions with NMP Lithium Ion batteries
>Date: Fri, 7 May 2004 02:30:32 -0500
>
>I'm not trying to start a "heated" debate about the safety of lithium 
>batteries, but I purchased a couple of these packs just before the AMA 
>warning and now I have some concerns and questions.
>
>My question revolves around how the NMP batteries are built and how the NMP

>charger works.
>
>In the past I'd believed that fires with lithium batteries are primarily 
>due to user error when charging, typically one of the following errors: 1) 
>Using the wrong type of charger, 2) setting the charger for the wrong 
>battery type/cell count, or 3) trying to charge cells too quickly, in 
>excess of 1C.
>
>However, I did some research on E-Zone and found that these packs can catch

>fire even on a slow charger if they are charged beyond 4.2 volts per cell. 
>This means if your charging a multiple cell pack and cells differ in 
>voltage by more than 1 volt and the charger keeps charging until (total 
>cells x 4.2 volts) is reached then even slow chargers can cause a fire.
>
>Therefore, my question is does circuitry exist either in the NMP battery 
>pack or in the NMP charger to assure that a single cell will never be 
>charged in excess of 4.2 volts? If so, how does this work?
>
>If this circuitry exists then we can probably feel quite safe leaving the 
>NMP batteries inside our planes while charging just as we do with our 
>laptops, cell phones, cameras etc. If not then we're taking a big chance 
>charging inside our planes, vehicles and houses (as has been pointed out on

>this list before).
>
>FYI, I was shocked when I visited E-Zone and found that the electric guys 
>have known for a long time that fires are not only a possibility but have 
>occurred many times. In fact, it's common knowledge in the electric 
>community that precautions such as charging in a flame proof container, not

>charging in vehicles or homes, never leave the battery unattended, etc. 
>must be observed when charging.  I found it very interesting that there are

>even numerous people working on various types of fire-proof charge bags 
>made from things like ceramic fibers.
>
>Obviously the electric guys push things to the extreme with their 
>batteries, but if such a simple thing as exceeding 4.2 volts can catch a 
>cell on fire then we should make sure our batteries/chargers have circuitry

>to prevent this or we must take the necessary precautions when charging.
>
>
>Here's a link to a good Lithium safety primer on E-Zone:
>http://www.rcgroups.com/forums/showthread.php?t=209187
>
>Thanks to anyone who can share the details of the NMP lithium system.
>
>Keith Black


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